The present invention pertains in general to circuits which produce a capacitance across a pair of terminals wherein the capacitance remains constant as a voltage signal across the terminals varies and, in particular, to circuitry for maximizing the bandwidth of a transconductance amplifier while minimizing overshoot. The present invention also provides for adjusting the value of capacitance across the terminals of the circuit.
Transconductance amplifiers are high precision wide band amplifiers which have broad application in modern electronic equipment. The transconductance amplifier is an amplifier in which a voltage is supplied as an input and a current results as an output. Further circuitry then produces an output voltage based on this output current. It is desirable to maximize the bandwidth of the amplifier as much as possible. It is known in the prior art that this can be achieved by adding a capacitor C1 across the gain setting resistor R1 as shown in FIG. 1. In terms of LaPlace transforms this addition of the capacitor C1 has the effect of adding a zero to the numerator in the LaPlace transform for the amplifier. As a result the frequency response to the amplifier is shifted from the curve 10 to the curve 12 shown in FIG. 2. This approach however also introduces overshoot 14 on the frequency response curve 12.
Furthermore, an additional problem in fabrication techniques occurs when the capacitance is provided across the gain setting resistor. When the transconductance amplifier is formed as an integrated circuit chip, the characteristics of the amplifier vary from one chip to the next. Thus it is necessary to use different values of capacitance for each amplifier. In the prior art, one method is to provide a capacitor on a printed circuit board which may be manually removed and replaced with capacitors of different values until the appropriate results are achieved. A drawback with this technique is that a series inductance is introduced between the capacitor and the resistor when the rest of the transconductance amplifier is part of the integrated circuit chip while the capacitor is a separate part on the printed circuit board. The series inductance produces undesirable effects in the circuit.
The present invention overcomes these drawbacks in the prior art.